•This provides a systematic, comprehensive review of two-phase instabilities.•Relevance to both macro- and micro-channels is carefully addressed.•Significant mechanistic differences among instabilities are addressed.•Confusion among studies over dominant instabilities is assessed. Study of two-phase flow instabilities began in the late 1920′s, and in the nearly 100 years since, significant progress has been made in both experimental and theoretical understanding of them. Despite these advances, many key deficiencies remain, solution of which will provide appreciable value for system designers looking to leverage phase change heat transfer technologies in a safe and repeatable manner. The present review provides a systematic overview of all key two-phase instabilities focusing on the fundamental mechanisms leading to their occurrence. Emphasis is placed on how these mechanisms may change depending on whether flow may be classified as macro- or micro-channel, particularly relevant due to the modern proliferation of parallel micro-channel heat sinks. Extensive literature surveys are performed for each instability type, and strengths and weaknesses of existing literature assessed. Focus is placed on providing recommendations for future work based on the status of current literature. Important takeaways include the significant mechanistic differences for Density Wave Oscillations and Parallel Channel Instability between macro- and micro-channels, the need for better understanding of the role of parallel micro-channels on external pressure curves (impacting Ledinegg instability and Pressure Drop Oscillations), and the influence of size and position of compressible volume on Pressure Drop Oscillations.